1. Field of the Invention
The present invention relates generally to processes for controlling the emission of hydrogen sulfide and nitrogen oxides from, and the growth of organisms, such as bacteria and algae, in, open cooling towers and the like in which sulfide- and nitrite-containing waters are cooled.
2. Background Discussion
General processes by which geothermal brine can be used to generate electric power have, of course, been known for some time. Geothermal brine, having a wellhead temperature of over about 400.degree. F. and a wellhead pressure of over about 400 psig, for example, can be flashed to a reduced pressure to convert some of the water or brine into steam. Steam produced in this manner is generally used in conventional steam turbine-type power generators to generate electricity.
Cooler, less pressurized, geothermal brine can be used in a closed-loop, binary fluid system in which a low-boiling point, secondary liquid is vaporized by the hot brine. The vapor produced from the secondary liquid is then used in a gas turbine-generator to generate electricity. The vapor from the turbine is recondensed and reused.
In both instances, the "used" geothermal brine is most commonly reinjected into the ground to replenish the aquifer from which it was extracted and to prevent ground subsidence. Reinjection of geothermal brine is also important to avoid the problems associated with the disposal of the large amounts of saline and usually highly-contaminated brine involved.
One of the many problems which has added significantly to the overall cost of producing electric power by the use of geothermal brines, relates to the emission of hydrogen sulfide. Although the amount of hydrogen sulfide contained in the separated/extracted steam and gas mixture usually varies from one brine source to another, levels of at leasts about 50 PPM (parts per million) are common. As an indication of the magnitude of this emission problem, at an assumed hydrogen sulfide concentration (in the steam) of about 50 PPM and for an assumed steam production rate of about 200,000 to 220,000 pounds per hour (the amount of steam typically obtained from a brine flow of about a million pounds an hour), nearly 50 tons of hydrogen sulfide gas are generated each year as an unwanted by-product.
Another problem which has added to the overall cost of producing electric power by the use of geothermal brines relates to the undesirable emission of nitrogen dioxide (NO.sub.2) The nitrogen dioxide results from the oxidation of ammonia in the brine by naturally occurring bacteria therein which produce nitrites. Nitrites can also produce acids which are quite corrosive to the cooling system and disposal piping.
It is important that any new process for controlling nitrogen oxide and/or hydrogen sulfide emissions from, and the growth of organisms in, steam condensate handling portions of geothermal brine power plants not only be effective to avoid penalties for excessive emissions, but that it also be economical. If a process is effective for controlling such emissions and organism growth but is uneconomical, the process is of little practical use in an actual geothermal brine power plant.